1. Field of the Invention
This invention relates generally to marine seismic cables, and more particularly to a marine seismic cable with selectively controllable buoyancy.
2. Description of the Related Art
Offshore oil and gas exploration and production operations frequently involve the repeated deployment of submerged cables for transmitting data signals and electrical power between various sensors and other electronic equipment and ship or land-based receiving stations. Marine seismic operations involve the static deployment of a submerged cable on the ocean bottom, or the suspension thereof between buoys or land structures. Other typical operations involve the towing of a submerged cable or "streamer" behind a ship. Still others employ a marine cable used as a tether for a Remotely Operated Vehicle ("ROV"). Such ROV set ups are now commonly used to conduct underwater inspections of various structures, such as oil rigs and the like.
Most conventional marine seismic cables consist of a tubular outer jacket that encloses a plurality of individually insulated conducting wires and one or more tension members that are typically placed on the center axis of the cable to restrict the elongation of the cable. These types of conventional cables are fabricated to have a fixed buoyancy at a preselected depth, namely, the anticipated depth of operation for the particular undersea operation. The design neutral buoyancy depth for a given conventional marine seismic cable is based upon an assumed density of sea water. There are a number of disadvantages associated with this type of design.
The operating depth of the fixed buoyancy cable is a function of the density of the ambient water, which is a function a number of parameters, such as temperature, salinity and mineral content, to name a few. Accordingly, where the ocean conditions encountered by the conventional fixed buoyancy marine seismic cable vary from the anticipated norm, this type of cable can deviate significantly from the desired depth. Furthermore, where operational needs dictate transition to another depth, a fixed buoyancy cable may have to be retrieved to the vessel and replaced with another cable, resulting in costly down time and the expense of acquiring and stowing additional cables onboard. Another disadvantage associated with conventional fixed buoyancy marine seismic cables is the potential for the enclosed conductors to be damaged during deployment and retrieval from the cable vessel. Most conventional seismic cables are deployed from a spool or supply stack that is mounted on the cable vessel. The cable is fed from the spool or supply stack, through a linear cable engine consisting of a plurality of opposed rotating tires, and over a sheave that is typically mounted near the stem of the cable vessel. As the cable passes over the sheave, the cable undergoes significant bending and may undergo significant tensile forces, depending upon the amount of cable in the water, the vessel speed, and sea conditions. This bending in conjunction with large tensile forces can cause the elongated tension members enclosed within the cable jacket to compress some of the enclosed conductors against the portion of the cable jacket that is in contact with the sheave against each other, possibly resulting in damage to those conductors.
Some conventional marine seismic cables contemplate variable buoyancy. In one design, a pair of opposed fluid lines are positioned in an expanded sleeve around which an outer sleeve and another sleeve are concentrically disposed. The conductor wires for this cable design are positioned between the expanded sleeve and one of the two outer sleeves. The fluid lines are provided with a preset amount of an oil and are interconnected via bypass valves which open and close in response to increases in pressure of the fluid as a result of water pressure bearing against the external cable sleeve. The cable is designed to change buoyancy automatically in response to encountering variations in water density. One difficulty associated with this conventional design is the fact that the conductor wires are directly exposed to forces imparted by the compression of the external cable jacket. Furthermore, the conductor wires are positioned very close to the exterior of the cable. Thus, the wires will be subjected to a significant bending and other stresses as the cable passes over the sheave during deployment and retraction from the cable vessel. In addition, this conventional design provides only a limited capability to provide variable buoyancy to the cable.
In another conventional variable buoyancy design, a tubular external jacket is provided with a relatively spacious internal cavity in which a bundle of signal conductors are positioned along with one or more strain members and a fluid supply line. The cable is provided with a pressure sensing switch that is housed eccentrically within a block that is concentrically disposed within the outer jacket. The switch assembly includes a bellows arrangement that is longitudinally movable to activate electrical switches which open and close valves enabling fluid to flow in and out of the supply line. The bellows arrangement is exposed to the ambient sea water such that changes in pressure associated with undesired changes in depth activate the bellows arrangement so that fluid is transferred into or out of the supply line as desired. Little structure is provided in this design for protecting the various conductors from the deleterious compressive forces associated with pressure induced compression of the external jacket as well as bending over the deployment sheave on the cable vessel. Furthermore, the eccentrically disposed pressure sensing apparatus is susceptible to damage during translation over the deployment sheave.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.